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Astron. Astrophys. 318, 947-956 (1997)

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1. Introduction

In the deep photosphere, particles are well coupled by collisions. That is why the physics of magnetic flux tubes is often described by the resistive one-fluid MHD approach. However a treatment of the atmosphere as an ensemble of three fluids (ions, electrons and neutrals) is necessary to give a clear physical insight on the mechanisms of current generation in these flux tubes. Moreover, higher in the solar atmosphere significant effects arise due to the density decrease that leads to a decoupling of motions of ions and neutrals, that cannot be described by the one-fluid approximation.

For an axially symmetrical magnetic field, the velocities of electrons, ions and neutrals in the photosphere have been found analytically in Hénoux and Somov (1991), referred as Paper 1 in what follows, by solving the equations describing the balance of horizontal forces. A quantitative model of the photospheric flux tube is obtained. Then a model of the chromospheric part of the flux tube is presented showing the effects of the electromagnetic and pressure forces generated by the current system. This model is still semi-quantitative. However, it allows us to relate the DC currents in flux tubes to the photospheric velocity field and demonstrates the significant rôle of these currents in the physics of flux tubes.

The plan of the paper is the following. In Sect. 2 we show that an influx of matter and angular momentum inside the flux tube at photospheric level generates azimuthal and radial current densities. In Sect. 3 the vertical electric current, which keeps the flux tube in a pinched state in the chromosphere, is computed together with the vertical current density profiles. We discuss the photospheric upflows generated by the electromagnetic forces inside the flux tube in Sect. 4. Vertical currents produce large effects at chromospheric heights; they are presented in Sect. 5. Next, we discuss in Sect. 6 three main consequences of the electromagnetic forces generated by DC currents flowing in the flux tube: coronal abundance anomalies, formation of chromospheres, and acceleration of spicules. Conclusions are given in Sect. 7.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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